The HD-MAC is an European HDTV system which is compatible with a MAC system and can transmit an image through direct satellite broadcasting, and is a TV system having a specification such as an aspect ratio of 16:9, 1250 scanning lines, a field rate of 50 Hz and a 2:1 interlaced scanning. Accordingly, in the HD-MAC system, scanning lines of odd fields and even fields exist alternately.
A simple way for searching a motion vector in an image which exists at different positions in adjacent fields is to estimate a motion vector between frames (for example, odd and odd fields, even and even fields).
However, the accuracy in the vertical directional motion vector is low because the motion vector in the HD-MAC system is for field-interpolation. However, artifacts (for example, line crawling and interline flicker) by interlaced scanning can be considerably reduced by means of a scanning conversion into a progressive scan scheme.
Accordingly, it is needed for an image signal having 1250 scanning lines/50 Hz field rate/2:1 interlaced scanning to be converted into an image signal having 1250 scanning lines/50 Hz rate/1:1 progressive scanning. This is also needed for block-interpolation in the HD-MAC decoder.
In order to convert an image signal of interlaced scanning type into an image signal of progressive scanning type as described above, an image signal of 2:1 interlaced scanning having a 40 msec Nyquist space is obtained by filtering of a 40 msec filter within the HD-MAC, and then pixels to be interpolated with a median filter must be extracted.
FIG. 1 is a schematic circuit diagram of a known Finite Impulse Response(FIR)-median hybrid filter, which circuit is constructed with a sliding window 12 for sliding-outputting an input image signal X.sub.k (X.sub.k =S.sub.k +N.sub.k) into a window (W=2P+1) of a predetermined size, first and second linear filters 14 and 16 for respectively outputting filtered signals L and R by respectively filtering the signal outputted from the sliding window 12 with the following equations (1) and (2), wherein ##EQU1## a median filter 18 for receiving the signals L and R respectively outputted from the first and second linear filters 14 and 16, and also the signal outputted from the sliding window 12, and outputting a median signal Y.sub.k. Here, S, N and P are signal level, noise level and number of pixels, respectively.
However, the conventional filter constituted as shown in FIG. 1 does not have an edge detection circuit and it has a mixed type structure composed of left and right linear filters 14 and 16 and a median filter 18 with respect to a center sample of an image signal. In such a circuit, as shown in FIG. 1, when noise of an input image signal is impulse, the L and R values respectively outputted from the linear filters 14 and 16 are values smeared out at the signal edge. That is, since the circuit as shown in FIG. 1 has no edge detection step, it cannot reduce the edge shifting effect due to the existence of impulse noise near the signal edge.